It is well known in the transit vehicle art to open and close transit vehicle doors by rotating electric motors. In a typical transit vehicle door system a reversible electric motor turns a gear drive which in turn rotates a drive screw. A drive nut mounted on a door hanger is driven by the drive screw to open and close the door.
The rotation and the angular position of the shaft of the motor is, in some systems, sensed by an incremental optical encoder which has a rotor attached to the motor shaft and coaxial with the motor shaft. The rotor has indicia disposed in the angular direction around the rotor.
The indicia, for example, may be opaque angular segments separated by transparent angular segments. These segments may be formed on a flange which is coaxial with the motor shaft. An optical encoder may have a light emitting diode on one side of the rotor and a photoreceptor such as a photodiode on the other.
A single light emitting diode and photoreceptor provide a first pulsed binary signal as the motor rotates. A count of the pulses provides an indication of the angle through which the motor has rotated.
In order to determine the direction of rotation, a second photoreceptor is also generally employed. It observes the first light emitting diode or a second light emitting diode. It is positioned so that it provides a second pulsed binary signal which is in quadrature relationship to the first pulsed binary signal.
In a typical optical encoder, for example, there are 500 transparent segments each having an angular extent of 0.36 degrees. Likewise there are 500 opaque segments, each having an angular extent of 0.36 degrees. The opaque segments alternate with the transparent segments. Each transparent segment and its following opaque segment together occupy an angle of 0.72 degrees.
To obtain the quadrature signal, the second photoreceptor is located at an angular position of (N*0.72)+0.18 degrees relative to the first photoreceptor, N being any integer.
When the motor rotates in one direction, pulses obtained from the first photoreceptor precede the pulses from the second photoreceptor. When the motor rotates in the opposite direction, pulses from the second photoreceptor precede the pulses from the first photoreceptor. A signal processor or logical device which controls the motor is programmed to utilize the pulses from the first and second photoreceptors to determine the direction of rotation of the motor, as well as its angular velocity and, by summation, calculate the angle through which it has rotated. In this manner, the signal processor or logical device continually has information defining the position and velocity of the door.
Some commercially available encoders also have a third photoreceptor which observes a light emitting diode. These pass light through a portion of the rotor having, for example, one small transparent angular segment and one opaque angular segment covering the rest of the 360 degrees. These provide one index pulse per revolution of the rotor. This single index pulse is used to verify the position obtained from the quadrature signals.
Optical encoders are subject to deterioration with time, making their output susceptible to electrical noise, which may make the edges of the pulses uncertain. There may also be crosstalk between the signals obtained from the photoreceptors. In addition, there may also be spurious signals on the lines from the photoreceptors due to electromagnetic interference.